Ubuntu – Linux software RAID that uses system RAM for write cache

That kind of depends on what service is providing the NAS functionality. Samba? NFS? All of the above? At its very base, Linux will use all available unallocated memory to serve as the file cache. That amount of memory will be used by default through the normal cache mechanisms of linux. Especially if a 64-bit kernel is used.

16GB is quite a lot of RAM for that problem. And yet, it can be just right for what you need. It all depends on how much of your data is in active use at any given time. If your 'working set' of active/open files is over 12GB, then 16GB of RAM is perfect. Ideally you want to have all of the open files able to fit into server-cache in order to provide maximum performance. What level that's at depends on your environment so there isn't any set answers.

It's good to have all that data fit in RAM for several reasons, but one of which is for writes. It allows the server to do I/O reordering to minimize HD latency, something that the RAID card will also do.

I can't find a anything that says if you create a RAID10 array of 4 devices then the mirrors will be (ab) (cd) or whatever. If you want to guarantee the placement then you can create a nested raid by first of all creating two mirrors then creating a stripe with them.

Create the mirrors

mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdf1
mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sdg1 /dev/sdh1

Then stripe them

mdadm --create /dev/md10 --level=0 --raid-devices=2 /dev/md0 /dev/md1

So now /dev/md10 is a stripe of two mirrors and you know the location of each type of disk.